Photocatalytic material and photocatalytic article

ABSTRACT

A photocatalytic material having titanium oxide crystals and anions X incorporated therein, which is prepared by at least one of a method comprising substituting anions X for some of the oxygen sites of titanium oxide crystals, a method comprising doping anions X between lattices of a titanium crystal and a method comprising doping grain boundaries of titanium oxide, or a combination of these method. The photocatalytic material has acquired a new energy level formed in a band gap of titanium oxide, which results in its exhibition of a photocatalytic activity by absorbing visible lights. The photocatalytic material can thus exhibit a satisfactory photocatalytic activity under sunlight and also in a room with a fluorescent lamp.

TECHNICAL FIELD

The present invention relates to a photocatalytic material and aphotocatalytic article employing a TiO₂ crystal system.

BACKGROUND ART

Hitherto, known materials exhibiting a photocatalytic action include thelikes of TiO₂ (titanium dioxide), CdS (cadmium sulfide), WO₃ (tungstentrioxide), and ZnO (zinc oxide). These photocatalytic materials aresemiconductors, absorb light to form electrons and holes, and presentvarious chemical reactions and bactericidal actions. However, becausetitanium oxide is nontoxic and is superior from the standpoint ofstability to water and acid, so far only titanium oxide has been put topractical commercial use as a photocatalyst.

However, because of the values of the band gap (Eg=3.2eV) of titaniumoxide, the operating light of such a titanium oxide photocatalyst islimited to ultraviolet light with a wavelength λ<380 nm. As aconsequence, there remains an unfulfilled demand for development ofmaterials which exhibit catalytic activity when irradiated with visiblelight with a wavelength of 380 nm or longer. These materials aredesired, for example, for use indoors and for improving photocatalyticactivity.

As described in Japanese Patent Laid-Open publication No. Hei 9-262482,by modifying materials using ion implanting of metal elements such as Cr(chrome) and V (vanadium) in anatase type titanium oxide having a highcatalytic activity, the light absorbing edge of titanium oxide can beshifted to the long wavelength side to permit the operation of titaniumoxide catalyst in visible light. No reports discussing the doping of Cr,V, and so on have been published since the early 1970s which succeededin operating under visible light. Japanese Patent Laid-Open publicationNo. Hei 9-262482 describes that operation under visible light can beenabled through use of special techniques for doping Cr, V, and so on.

Thus, in the above conventional example, the operation of TiO₂photocatalyst under visible light is made possible by a technique of ionimplanting metal elements in TiO₂. However, metal ion implantation isdisadvantageous because of its high cost. While there is a demand formethods for manufacturing TiO₂ photocatalyst, such as by synthesis insolution or by sputtering, when these methods are employed, theresulting photocatalysts can not be operated under visible light. It isgenerally considered that this is because Cr of the dopant aggregates orforms oxides such as Cr₂O₃ in a crystallization process. Thus, in theconventional examples, there is a problem that a technique of ionimplanting metal elements must be adopted in order for metal elements tobe used to enable operation of TiO₂ under visible light.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a photocatalystwhich can operate under visible light by using novel materials andwithout using expensive techniques such as ion implantation.

In view of the above situation, the present inventors realized thepresent invention after conducting theoretical study of state densityand optical physical properties using the first principle calculation aswell as experimental study of photocatalysts reacting to light in awavelength region extending from ultraviolet through visible light.

That is, the photocatalytic material according to the present inventioncomprises titanium compound Ti—O—X obtained by at least one ofsubstituting an anion X at a plurality of oxygen site of titanium oxidecrystals, doping an anion X between lattices of a titanium oxidecrystal, and doping an anion X in grain boundaries of polycrystallineaggregate of titanium oxide crystal.

As a product of the above, a photocatalyst which exhibits photocatalyticactivity after absorbing visible light can be obtained. Thus, thephotocatalyst can exhibit satisfactory photocatalytic activity evenunder solar or fluorescent light.

Moreover, it is preferable in the Ti—O—X of the present invention thatthe anions be an element or molecule containing at least one of B, C, P,S, Cl, As, Se, Br, Sb, Te, and I. With these anions X, a new energy bandis formed within the band gap of titanium oxide, permitting absorptionof visible light.

Still further, it is preferable in the Ti—O—X of the present invention,that there be a chemical bond between titanium Ti and anions X. Thisresults in charge-transfer between Ti and X and formation of an energyband, permitting efficient absorption of visible light.

Furthermore, it is preferable that TiO₂ crystals are formed on theexternal surface side of Ti—O—X of the present invention. With such aconfiguration, internal photocatalytic materials are able to absorbvisible light to produce electrons and holes, with a result thatphotocatalytic action is exhibited by the TiO₂ crystals at the surface.The resulting photocatalyst can use visible light as operating lightwith maintaining functionality similar to conventional TiO₂photocatalysts. For example, this constitution is very advantageous fordecreasing the contact angle of water to realize a hydrophilic property.

Moreover, it is suitable that the Ti—O—X of the present invention bemainly oriented along the C axis direction at its surface. Thisconstitution permits efficient light absorption at the surface becauseof anisotropy of optical absorption characteristics of photocatalyticmaterials.

In addition, as the crystalline phase of Ti—O—X to realize thesecharacteristics, any combination of single crystals, polycrystals, oramorphous Ti—O—X may be used. However, single crystals and polycrystalstend to exhibit a greater photocatalytic activity than does amorphousTi—O—X.

Further, in the Ti—O—X photocatalyst of the present invention, anycrystal form of anatase, rutile, and brookite may be employed as thebasic crystal.

BRIEF DESCRIPTION DRAWINGS

FIG. 1 is a view showing the state density of Ti—O—X.

FIG. 2 is a view showing the level of the p state of various atomsmeasured from the level of the 2p state of O atom obtained by usingFLAPW process.

FIG. 3 shows the constitution of an Embodiment 1.

FIGS. 4(a) and 4(b) are views showing the crystalline phase of titaniumoxide.

FIGS. 5(a) and 5(b) are views showing the energy dependency of theimaginary part of a dielectric constant function.

FIGS. 6(a) and 6 (b) are views showing an Embodiment 3 of gradientcomposition.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment 1

From a theoretical study of electronic state, density of state, andoptical characteristics using the first principle calculation, thepresent inventors realized that the titanium oxide semiconductorcontaining anions X of the present invention forms a new level (energyband) contributing to absorption of visible light in the band gap oftitanium oxide.

More specifically, electronic state and optical characteristics wereevaluated using TiO_(1.75)X_(0.25) unit cell substituting anions X forsome of the oxygen sites of anatase titanium oxide by FLAPW(full-potential linearized-augumented-plane-wave) process, which is aprinciple calculation method.

FIG. 1 shows density of state (DOS) values calculated for semiconductorTi—O—X. In this example, F, N, C, B, S, and P were adopted as anions Xand the calculation results for these materials are shown.

Thus, it was found that the position at which the impurity level isformed varies with substitution species and correlates with the ionicityof these substitution species. Here, a valence band that rises on theminus side from energy 0 eV in each density of state in this FIG. 1, aconduction band of titanium oxide rises up on the plus side from thevicinity of 2.5 eV, and the interval between them corresponds to a bandgap. Because the reduction level of water is in the vicinity of theconduction band of titanium oxide, it is desirable to design a narrowband gap by raising the valence band towards the conduction band oftitanium oxide by forming the new impurity level at around the valenceband, rather than the around conduction band, of titanium oxide.

Hence, it is found that nitrogen N and sulfur S are desirable as anionsX from the standpoint of smooth hybridization of impurity levels and thetitanium oxide band. However, in addition, it is found that visiblelight operation is possible also for boron B, carbon C, and phosphorus Pbecause absorption takes place in the visible light region.

Further, the results of these density of state studies confirmed thatapproximately similar results are obtained in a unit cell containinganions X at lower concentration such as TiO_(1.88)X_(0.12) and so on.

The variation of the electronic state of titanium oxide due tosubstitution of these anions X is mainly attributed to the difference ofatomic levels between O and anions X for titanium atom. FIG. 2 shows thelevels of the p state of various atoms measured from the levels of the2p state of O atom obtained using the same FLAPW process. A comparisonwith the results shown in FIG. 2, considering that the valence band oftitanium oxide is mainly formed by the O 2p state in the density ofstate of FIG. 1, reveals that the impurity levels for the band oftitanium oxide are qualitatively consistent with the atomic level of thep state. That is, in both cases the order of B→C→N→(valence band)→F ismaintained and the levels of C and P and those of N and S appear nearthe same respective positions.

Then, also for Cl, As, Se, Br, Sb, Te, and I, the levels are present atnearer positions. Therefore, it can be seen from the results shown inFIG. 2 that visible light operation is possible because absorption inthe visible light region similarly occurs also for Cl, As, Se, Br, Sb,Te, and I as anions X.

As described above, it is important that a Ti—X bond for charge-transferis present between Ti and X in order that the position of the atomiclevel of anions X for titanium atom is reflected in an impurity level.Then, the anions X may be those substituting oxygen as well as those inwhich anions X are present in crystal lattices in an interstitial siteor crystalline boundaries, or a combination of these. Moreover, anioncontaining effects of the present invention can be obtained when anionsX are single elements such as B, C, P, S, Cl, As, Se, Br, Sb, Te, and I,or a combination of these elements, or in a molecular state containingoxygen and hydrogen.

Furthermore, although in the above example anion containing effects werecalculated for the crystal lattice of anatase type titanium, similareffects can be obtained with rutile, brookite, and amorphous titaniumoxides containing anions.

Here, FIG. 3 is a view showing the constitution of an Embodiment 1 inwhich Ti—O—X film 12 of a photocatalytic material is formed on a SiO₂substrate 10. This Ti—O—X film 12 is prepared by at least one of amethod comprising substituting anions X for some of the oxygen sites oftitanium oxide crystals, doping anions X in an interstitial site oftitanium oxide crystal, doping anions X in grain boundaries ofpolycrystalline aggregate of titanium oxide crystal, or a combination ofthese methods.

FIG. 4(a) shows rutile type titanium oxide crystals and FIG. 4(b) showsthe crystalline unit lattices of anatase type titanium oxide. In thesefigures, small and large ◯ show Ti and O, respectively. Ti—O—X is formedby substitution of X for a part of this O or by doping anion X in theinterstitial site within crystals or grain boundaries of titanium oxidecrystals.

An example manufacturing process for producing such a photocatalyticmaterial will be described. In this example, Ti—O—X film 12 ismanufactured by RF magnetron sputtering.

SiO₂ substrate 10 and titanium oxide target (or Ti—X, target for exampleTi—S) are set in the vacuum chamber of a RF magnetronsputtering device.Then, an appropriate amount of gas containing anions X (for example,SO₂+O₂), gas, and an inert gas (for example Ar gas) are introduced intothe vacuum chamber to conduct sputtering. Ti—O—X film 12 is accumulatedon SiO₂ substrate 10. Moreover, as the substrate 10, various materialssuch as ceramic can be utilized.

Further, after deposition of Ti—O—X film 12 by sputtering, heattreatment (annealing) is performed for crystallization. Although simplefilm deposition yields an amorphous structure containing polycrystals,it is possible by heat treatment to attempt poly- andsingle-crystallization and further to form chemical bonds betweentitanium and anions X. Moreover, heat treatment after depositing filmmay also be omitted by forming Ti—O—X film 12 with heating the SiO₂substrate 10.

Furthermore, although in the above example, Ti—O—X as a photocatalyticmaterial in the form of a thin film was described, Ti—O—X can be appliednot only in a thin film but can also be included in binder materials forpainting such as silica, alumina, fluororesin (polytetrafluoroethylene),those containing nitrogen, and compound complexes of them in which finegrain Ti—O—X based Ti—O—X is mixed and in silica, alumina, fluororesinor those containing nitrogen, or those containing nitrogen, or compoundcomplexes of them used as internal base materials, on the whole surfaceof which, or on some portion of the external surface of which, Ti—O—X isformed.

Furthermore, it is possible for Ti—O—X to be prepared by various methodsof preparing fine grain, a sol gel method, and a chemical reactionmethod with the above manufacturing process as a base.

Embodiment 2

FIG. 5(a) and FIG. 5(b) show energy E (eV) dependency of the imaginarypart of dielectric constant function (e2xy, e2z) obtained bycalculation. Here, FIG. 5(a) shows energy dependency in the xy direction(vertical direction to C axis) for titanium oxide crystals, while FIG.5(b) shows dependency in the z direction (C axis direction).

This imaginary part of dielectric constant function corresponds to thewavelength dependency of optical absorption characteristics. In bothTi—O—N and Ti—O—S, absorption ends are shifted to the lower energy side,that is, the side of wavelengths longer than those of titanium oxide.This result indicates that visible light operation is possible byperforming substitution of one of or both of N and S in titanium oxide.Moreover, it was found from the difference between FIG. 5(a) and FIG.5(b) that optical anisotropy is strong in both titanium oxide and Ti—O—X(X═N or S), and it was found from this result that dependency ofphotocatalytic activity on a crystal face is strong.

Thus, the absorption end especially in the xy direction is noticeablyshifted to the visible light region. It can be seen from this that thephotocatalytic article Ti—O—X of the present invention is suitable whenthe surface structure is mainly oriented in the direction along a Caxis. Because light vertically incident to the surface has components ofan electric field in propagation and vertical direction (the directionvertical to the surface), visible light can be efficiently absorbedbecause of light absorption characteristics in the xy direction in FIG.5(a) if the surface is oriented along the direction of the C axis.

Embodiment 3

FIGS. 6(a) and (b) show the constitution of an Embodiment 3. In FIG.6(a), Ti—O—X film 12 is formed on the SiO₂ substrate 10, and TiO₂ film14 is formed thereon. Moreover, although in the figure, laminatedstructure of two layers is shown, the boundaries of both becomeindistinct in the course of heat treatment or the like, resulting in aconstitution wherein S gradually decreases toward the surface. That is,TiO₂/Ti—O—X film of gradient composition is formed in which the densityof S atom is less nearer to the surface, and in which TiO₂ is exposed atthe outmost surface, though it is also possible to maintain a distinctboundary between the Ti—O—X and TiO₂ films.

Such a TiO₂/Ti—O—X film can be prepared, for example, as follows. First,as a target, Ti, TiO₂, or TiS₂ (titanium sulfide) is used and sputteringis conducted in SO₂+O₂+inert gas (for example, Ar) to form the Ti—O—Xfilm. Subsequently, TiO₂ film is prepared by deposition and heattreatment (for example, 550° C., two hours). Moreover, it is alsopossible to form TiO₂/Ti—O—X film of gradient composition by anothermanufacturing process. Furthermore, TiO₂/Ti—O—X film may be in granularform.

Moreover, gradient composition can be produced not only by heattreatment after lamination layer formation of Ti—O—X and TiO₂ films, butalso by changing the gas composition in an atmosphere according to thedeposition state of the film. That is, it is possible to form TiO₂ onthe surface side by gradually decreasing the N₂ partial pressure of theatmosphere during deposition.

Ti—O—X is a semiconductor which absorbs visible light to produceelectrons and holes and exhibits a photocatalytic activity through useof visible light as an operating light. Accordingly, the photocatalystof this embodiment of gradient composition TiO₂/Ti—O—X exhibits aphotocatalytic activity similar to that of the TiO₂ film while usingvisible light as the operating light.

That is, visible light is absorbed in the Ti—O—X region (Ti—O—X film 12)near the substrate 10 to produce electrons and holes which are suppliedto TiO₂ (the TiO₂ film 14). In this manner, photocatalytic activity isexhibited at the surface of the TiO₂ film 14.

Hence, in the TiO₂ film, similar to the conventional example,photocatalytic activity is produced using visible light as operatinglight. Because comparison of hydrophilic property (contact angle θ) ofTi—O—X and TiO₂ films shows that the TiO₂ film is superior, improvementin hydrophilic property by the TiO₂ film may be sought under visiblelight. That is, with the configuration of this embodiment, it ispossible for hydrophilic property to be exhibited under irradiatingirradiation of only visible light and to improve the long-termhydrophilicity compared to that of the TiO₂ film.

Furthermore, it is suitable that the TiO₂/Ti—O—X photocatalyst ofgradient composition, as shown in FIG. 6(b), be in the form of a grainhaving Ti—O—X part 22 on the inside and TiO₂ part 24 on the outside. Itis suitable that such a photocatalyst in the form of a grain be mixed ina binder for paints and utilized like paint.

Moreover, the photocatalytic materials of the present invention may beutilized not only in the form of a thin film, but also in various formssuch as particles and particle-based binder materials. These variousforms of Ti—O—X can be prepared by methods such as various method ofpreparing thin films, various method of preparing particles, sol gelmethods, chemical reaction methods, treatment in plasma containing anionspecies X, and ion implantation of anions X, in addition to thesputtering method described in the example embodiments.

As explained above, according to the present invention, through a methodcomprising by at least one of substituting anions X for some of theoxygen sites of titanium oxide crystal, doping anions X in aninterstitial site of a titanium oxide crystal, doping anions X in grainboundaries of titanium oxide, or by a combination of these methods,anions X are introduced and trapped in titanium oxide crystals and, as aresult, a new level is formed in the band gap of titanium oxide. Thus, aphotocatalyst may be obtained which exhibits a photocatalytic activityunder visible light, and satisfactory photocatalytic activity can beobtained even under solar or fluorescent light.

INDUSTRIAL APPLICABILITY

Defogging and proofing against the effects of organic substancedecomposition can be obtained by forming the present invention onvarious surfaces.

What is claimed is:
 1. A photocatalytic material exhibiting aphotocatalytic action when exposed to light with a wavelength in theregion of ultraviolet light and visible light, comprising a titaniumcompound Ti—O—X obtained by at least one of: substituting an anion X fora plurality of oxygen sites of titanium oxide crystals, doping an anionX between lattices of a titanium oxide crystal, and doping an anion X ingrain boundaries of titanium oxide aggregate.
 2. The photocatalyticmaterial according to claim 1, wherein said anion X is at least oneelement selected from the group consisting of B, P, S, Cl, As, Se, Br,Sb, Te, and I, or a molecule containing at least one of these elements.3. The photocatalytic material according to claim 1, wherein a chemicalbond is present between titanium Ti and an anion X.
 4. A photocatalyticarticle comprising the photocatalytic material according to claim 1 anda titanium oxide crystal containing no anions X, formed on the externalsurface of the photocatalytic material.
 5. A photocatalytic articlecomprising the photocatalytic material according to claim 1, wherein thephotocatalytic material is oriented, on its surface, along the C axisdirection of the crystal.
 6. The photocatalytic material according toclaim 2, wherein a chemical bond is present between titanium Ti and ananion X.
 7. A photocatalytic article comprising the photocatalyticmaterial according to claim 2 and a titanium oxide crystal containing noanions X, formed on the external surface of the photocatalytic material.8. A photocatalytic article comprising the photocatalytic materialaccording to claim 3 and a titanium oxide crystal containing no anionsX, formed on the external surface of the photocatalytic material.
 9. Aphotocatalytic article comprising the photocatalytic material accordingto claim 6 and a titanium oxide crystal containing no anions X, formedon the external surface of the photocatalytic material.
 10. Aphotocatalytic article comprising the photocatalytic material accordingto claim 2, wherein the photocatalytic material is oriented, on itssurface, along the C axis direction of the crystal.
 11. A photocatalyticarticle comprising the photocatalytic material according to claim 3,wherein the photocatalytic material is oriented, on its surface, alongthe C axis direction of the crystal.
 12. A photocatalytic articleaccording to claim 4, wherein the photocatalytic material is oriented,on its surface, along the C axis direction of the crystal.
 13. Aphotocatalytic article comprising the photocatalytic material accordingto claim 6, wherein the photocatalytic material is oriented, on itssurface, along the C axis direction of the crystal.
 14. A photocatalyticarticle according to claim 7, wherein the photocatalytic material isoriented, on its surface, along the C axis direction of the crystal. 15.A photocatalytic article according to claim 8, wherein thephotocatalytic material is oriented, on its surface, along the C axisdirection of the crystal.
 16. A photocatalytic article according toclaim 9, wherein the photocatalytic material is oriented, on itssurface, along the C axis direction of the crystal.
 17. A photocatalyticarticle comprising the photocatalytic material according to claim 1,formed on a substrate.
 18. The photocatalytic article according to claim4, wherein the boundary between the photocatalytic material and thetitanium oxide crystal is indistinct, whereby the concentration of anionX gradually decreases toward the surface.
 19. The photocatalyticmaterial according to claim 1, which is in the form of a particle.